The basic mechanisms of nitrogen oxides (NO.sub.x) formation in combustion processes have been studied extensively and many methods are known for reducing NO.sub.x emissions from combustion sources. For reduction of "thermal NO.sub.x ", or NO.sub.x formed from the reactions of molecular nitrogen and oxidants, reduction of peak flame temperature is considered to be the key requirement. From the theoretical point of view, the lowest NO.sub.x emission can be obtained when the combustion reactions proceed homogeneously throughout the furnace at the lowest temperature permissible for the combustion process. In conventional practice, fuel and oxidant are introduced from a burner into a furnace to produce a stable flame where intense combustion reactions take place. When fuel and oxygen are post mixed, combustion reactions takes place at the boundaries of small fuel and oxygen fluid fragments created by turbulence. Local temperature at the flame front often approaches the theoretical flame temperature and hence a high level of NO.sub.x is formed.
One effective way for suppressing the flame temperature is to reduce the fuel and/or oxygen concentrations at the flame front by dilution with other non-reacting gases. Recirculation of flue gas by mixing with fuel or oxidant prior to the introduction of fuel or oxidant streams into a burner is an effective way to reduce NO.sub.x. In-furnace-dilution of fuel or oxygen by burner design is also practiced. Many known methods for reduction of NO.sub.x deal with improvements in the method of mixing fuel and oxidant at or near the burner to reduce the peak flame temperature. The NO.sub.x emission levels achieved by such known methods are still substantially above the lowest possible levels predicted for the ideal homogeneous reactor in which the temperature and species concentrations are uniform throughout the furnace and combustion reactions take place uniformly.
A combustion method which can enable low NO.sub.x generation, closer to the low ideal rate than can known combustion methods, is desirable.
Accordingly, it is an object of this invention to provide an improved combustion method wherein combustion can be carried out with low NO.sub.x generation.